igneous
Polished porphyritic volcanic rock, likely andesite or basalt
Porphyritic andesite/basalt; commonly plagioclase and pyroxene phenocrysts in a fine volcanic groundmass
AI-generated identification · may be incorrectDark olive-brown, wet-looking polish with scattered cream-white phenocrysts and possible rusty alteration. Hardness about 5.5–6.5 Mohs, fine-grained, compact, opaque, with no obvious cleavage; exact density and mineralogy require testing.
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Physical properties
Dark olive-brown, wet-looking polish with scattered cream-white phenocrysts and possible rusty alteration. Hardness about 5.5–6.5 Mohs, fine-grained, compact, opaque, with no obvious cleavage; exact density and mineralogy require testing.
Formation & geological history
An extrusive igneous rock formed when lava cooled relatively quickly after earlier growth of larger crystals. Its age cannot be determined from the image; similar rocks occur in volcanic arcs and basaltic lava fields worldwide.
Uses & applications
Primarily a decorative tumbled or polished specimen, with related andesite/basalt used as aggregate, paving stone, riprap, and dimension stone. It has little jewelry value unless unusually patterned or locality-specific.
Geological facts
The contrasting pale spots are likely feldspar or altered mineral crystals, producing a porphyritic texture. Brown-green coloration may reflect iron oxidation or chlorite/epidote alteration.
Field identification & locations
Identify by its tough, fine-grained volcanic texture and embedded lighter crystals; it is not typical jasper or agate because it lacks visible silica banding or translucency. Common counterparts are found in Mexico, the western United States, South America, and volcanic regions globally.
